ENERGY-DEPENDENCE OF DOUBLE PHOTOIONIZATION IN HE

The ratio of double-to-single ionization of He has been measured betwe en 280 eV and 1210 eV to investigate its energy dependence in the inte rmediate region. The new intermediate energy measurement, compared wit h the most recent theories of Pan and Kelly and of Hino and Hino et al ., show the importance of including not only ground state but also fin al state correlations. They also appear to indicate the importance of including higher-order effects in the theory. The previously reported [11, 22] high energy measurements between 2-12 keV give a ratio of 1.5 (2)% in good agreement with older shake calculations of Byron and Joa chain and of Aberg, as well as with recent many-body perturbation theo ry of Ishihara et al., calculations of Dalgarno and Sadeghpour, and of Andersson and Burgdorfer. In contrast to the intermediate energy beha vior, consideration of final state correlations proves inessential as discussed by Dalgarno and Sadeghpour.